They have to prove an intention to drive, being booked into the hotel, recovering a bag from the boot isn’t showing intention. You won’t get done.
no THEY don’t – you have to prove (but only on balance of probabilities) that: the circumstances were such that there was no likelihood of his driving the vehicle whilst the proportion of alcohol in his breath, blood or urine remained likely to exceed the prescribed limit.
I’d suggest that having booked a hotel room, having checked into that hotel room, having prearranged to leave your car there before checkin opened would all go a long way towards showing that. Ensuring you don’t use your “right to remain silent” when you have a clear defence is quite important. Certainly going to the boot not the drivers door is consistent with the defence but walking across the car park with the keys in your hand is technically enough to show the “drunk in charge” offence. A “no comment” interview (legally often a good idea when it’s their burden to prove something) will not help you. If you are drunk enough to do it whilst the cops stand watching they might have a point! I suspect most people who find themselves in court for drunk in charge for these sort of circumstances have failed at attitude test at the time: “where are you going? None of your business”.
I think the OP is probably being a bit paranoid – in general cops are dealing with drink driving for a specific reason (accident, phone call reporting it, or because they can clearly see you struggling to get into your car drunk) not because they are bored and want to go back to the warm police station. BUT in general people should probably think a bit more about whether they are in control of a car when it’s just sitting in a car park. Having a defence and having several months of hassle and stress going to court to establish that defence are not the same thing.
finally – if you will be that drunk by checkin time that a passing cop is likely to take interest, have you considered checkout Time too?